The present invention relates to an improvement in ophthalmic lens design and more specifically relates to such a lens which provides bifocal vision.
Presbyopia is a vision defect associated with advancing age. The presbyopic patient lacks visual accomodation, i.e. the ability of the eye to adjust to clearly see objects that are close to or at intermediate distances from the eye without the aid of a corrective lens.
The common correction for presbyopia is to use bifocal eyeglasses which have an upper portion ground for distance viewing, having, most commonly, a correction for myopia or hyperopia, and a lower portion with diopters added for near viewing. However, this solution does not readily lend itself to contact lenses, which tend to move with the eyeball and disrupt vision. To overcome this problem, bifocal contact lenses have been used which have a thick lower edge which is engaged by the lower eyelid when the wearer looks down, causing the contact lens to slide upwards on the cornea relative to the pupil. Such lenses which move relative to the eye are hard to fit because the lens must be sized properly to be engaged by the lower lid. Also, the amount of movement of the lens must be accurately measured to determine the desired height of the bifocal segment.
There have been a number attempts at providing bifocal lenses which avoid the problems of the above-described, thick edged lens. One group of attempts provides a multifocal diffractive lens constructed by means of a series of concentric, phase manipulating annular rings. The rings provide simultaneous focal powers for near and distant vision having sharp transitions. A second group of attempts relates to a multifocal ophthalmic lenses having a plurality of concentric rings with a continuously varying power within each zone as well as in transition from one zone to another. The zones are accomplished either by continuously changing curvature of the posterior surface of the lens, or by creating non-homogeneous surface. characteristics having refractive material indices which continuously vary in the lens radial direction.
A problem exists, however, in that the concentric ring configurations described above are closed and form sealed regions around the cornea. The central portion of the human cornea is a very important optical component to vision. Proper tear flow at the central portion is critical to corneal health. Contact lenses, especially those which are hydrophilic, conform to the cornea when worn. The closed concentric rings on the lens back profile isolates tear film in the central cornea into closed ring shaped channels. Consequently, the tears will only circulate in a small, local area and the cornea is not easily refreshed. Also, an increase in debris trapped under the lens, as well as in corneal indentation patterns, is likely to occur.
Therefore, there exists a need for a multifocal lens which eliminates the problems inherent in using concentric rings, and more particularly, which provides for proper tear flow to all parts of the central cornea.